From the textbook Neuroscience. 2nd edition:

Neuropeptides are relatively large transmitter molecules composed of 3 to 36 amino acids

 

Individual amino acids, such as glutamate and GABA, as well as the transmitters acetylcholine, serotonin, and histamine, are much smaller than neuropeptides and have therefore come to be called small-molecule neurotransmitters

Large proteins are around 10nm, because the amino acids wrap around each other.

This means that a good estimate of the size of neurotransmitters would be from around 0.5 to 5 nanometers.

Typical synaptic cleft distance is around 20 nanometers

From the textbook Neuroscience. 2nd edition:

Neuropeptides are relatively large transmitter molecules composed of 3 to 36 amino acids

Individual amino acids, such as glutamate and GABA, as well as the transmitters acetylcholine, serotonin, and histamine, are much smaller than neuropeptides and have therefore come to be called small-molecule neurotransmitters

Large proteins are around 10nm, because the amino acids wrap around each other.

This means that a good estimate of the size of neurotransmitters would be from around 0.5 to 5 nanometers.

Typical synaptic cleft distance is around 20 nanometers